The present invention relates generally to rotary machines, and, more particularly, to methods and apparatus to facilitate sealing between rotary and stationary components within a rotary machine.
Steam and gas turbines are used, among other purposes, to generate power for electric generators. Known steam turbines have a steam path that typically includes, in serial-flow relationship, a steam inlet, a turbine, and a steam outlet. Known gas turbines have a gas path which typically includes, in serial-flow relationship, an air intake (or inlet), a compressor, a combustor, a turbine, and a gas outlet (or exhaust nozzle). Compressor and turbine sections include at least one row of circumferentially spaced rotating blades or buckets.
Turbine efficiency depends at least in part on controlling a radial clearance or gap between the rotor shaft and the surrounding casing or outer shell. If the clearance is too large, steam or gas flow may leak through the clearance gaps, thus decreasing the turbine's efficiency. Alternatively, if the clearance is too small, the rotating packing seal teeth may undesirably contact the stationary packing seal or vice versa, during certain turbine operating conditions, thus adversely affecting the turbine efficiency. Gas or steam leakage, through the packing seals represents a loss of efficiency and is generally undesirable.
To facilitate minimizing seal leakage, at least some known turbines use a plurality of labyrinth seals. Known labyrinth seals include longitudinally spaced-apart rows of labyrinth seal teeth to facilitate sealing against pressure differentials that may be present in a turbine. However, certain off-normal operating conditions of the turbine may cause a flexure of the turbine casing, a bow in the rotor shaft, and other conditions that may cause the labyrinth seal teeth to contact other turbine components. Such contact, known as rubbing, may damage or distort the shape of the teeth and increase the clearance between the rotor and the casing such that the turbine thermal efficiency may be reduced. For example, temperature excursions during startup may distort turbine components, and result in the packing rubbing against the turbine shaft. Once the clearance between the shaft and the packing expands beyond original design specifications, efficiency losses due to steam leakage through the packing may increase. Generally, a damaged seal is only repairable or interchangeable during a turbine outage. Alternatives to known labyrinth seal designs may improve a seal's tolerance to rubs, however known designs may not be able to prevent rubs from occurring.